I'm sorry but there's not a magic solution to your question. Depending on the skin you have, you'll have to modify one or other thing. But this is nothing you can do from inside DNN. What you have to do is open the skin file (on \portals\_default\skins\NAMEOFSKIN or portals\PORTALNUMBER\skins\NAMEOFSKIN) and find where the panes are defined and how the widths are applied. Depending on the skin this can also be defined on the .css files. Look for files with extensions like .ascx, inside those look for something that looks like

runat="server" id="ContentPane"

This the definition of a pane. You'll have then to find how the widths are applied.
Hope this helps.

Thank you!